Digital processors are used in modern communication devices to perform complex processing while adhering to reasonable power and size constraints. In order to transfer information to another radio, digital signals containing the information are first converted to analog signals for transmission. This conversion process is performed by a digital-to-analog converter (DAC).
The frequency representation of a digital signal consists of an infinite number of replicas of the equivalent baseband analog signal. The replicas, also referred to as image spectra, are separated in the frequency domain by fs, which is the frequency of the digital sampling clock. All but one replica must be suppressed at the output of a digital-to-analog converter (DAC). Otherwise, due to the close frequency spacing of the replicas, non-linear action in an up-conversion mixer or power amplifier will result in inter-modulation distortion in the pass band.
FIG. 1 shows the frequency (image) spectrum 10 of three over-sampled digital signals 15a, 15b and 15c. Each of the three signals has been up-converted in the digital domain by a different frequency. In a conventional DAC, the desired images lie in the first Nyquist zone. In a radio frequency (RF) DAC, the desired images are in a different Nyquist zones, such as the 2nd Nyquist zone as indicated in FIG. 1. Two design challenges are presented by RF DACs. First, the desired images must not be significantly attenuated by a sample-and-hold (S&H) frequency response of the DAC. Second, the undesired images must be filtered out before the signal is sent to a power amplifier.
The first design challenge may be met with a S&H frequency response that contains a large magnitude near the sampling frequency (frequency=1 in FIG. 1.) Commercially available RF DACs can achieve this goal. For example, the curve 12 is a representative RF DAC frequency response that is nearly constant over the second Nyquist zone.
The second design challenge is met using analog bandpass filtering. A bandpass filtering requirement is shown by the dashed line 14 in FIG. 1. Note that the selectivity demands of the bandpass filter are high when desired image spectra are close to the edge of a Nyquist zone. As the selectivity demands of the analog bandpass filter increase, the filter order increases, and the passband phase distortion increases. This problem is exacerbated when very wideband signals or multi-frequency signals with wide frequency separations are to be transmitted, and the resulting extremely difficult filtering requirements require multiple cascaded filter designs.
Therefore, there is a need for suppressing image spectra of a desired frequency response so that a relatively low order bandpass filter may be used to select the desired frequency response.